Had a problem finding out where DHCP was coming for a number of my sites. One way of finding this out is to get someone on the user VLANs at each to send me the output of ipconfig /all, but that’s a hassle. Another way of doing it is to login into the routers and look at the running config (IP addresses mangled to protect the innocent), this’ll only work for certain Cisco devices:
1. Pick the VLAN of interest e.g. 172.xx.xx.0/24
2. Login into the associated router (layer 3 device) through the default gateway / HSRP address of the VLAN, in this case 172.xx.xx.1
3. Go to “Enable”
3. Do a “show run”
Where DHCP is configured on the router you’ll get the DHCP information used by the router:
ip dhcp pool EXAMPLE1
network 172.xx.x1.0 255.255.255.0
default-router 172.xx.x.1
netbios-name-server 172.1.2.11 172.1.2.12
dns-server 172.1.2.3 123.124.125.80
lease 14
Where DHCP is done through another server an IP helper address will be configured on the VLAN:
interface EXAMPLE2
description EXAMPLE2
ip address 172.xx.x.2 255.255.255.0
ip helper-address 172.1.2.3
no ip redirects
no ip unreachables
standby 25 ip 172.xx.x.1
standby 25 priority 110
standby 25 preempt
4. So in EXAMPLE1 DHCP is done from the router, in EXAMPLE2 DHCP is done from 172.1.2.3
There’s a couple of examples where this didn’t work but 90% of the time it gave me the answers I was looking for.
